In general, it is required to remove as much impurities, particularly artificial compounds, as possible from compounds to be taken into the human body such as pharmaceuticals. According to the bulk pharmaceuticals guideline by International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH), for example, the weight percent of impurities is to be 0.05% or less. L-Alanyl-L-glutamine is a dipeptide used as a pharmaceutical material (e.g., a component of an infusion preparation) and in cosmetics, and when used as a pharmaceutical material or the like, the above standard is applied.
Known examples of processes for producing dipeptides by the chemical synthesis method, for example, processes for producing L-alanyl-L-glutamine, include a process which comprises condensing N-benzyloxycarbonylalanine with a protective group-added glutamine and then removing the protective group (non-patent document Nos. 1 and 2), a process which comprises condensing N-benzyloxycarbonylalanine with glutamine without a protective group and then removing the protective group (patent document No. 1) and a process which comprises subjecting an N-(2-substituted)-propionylglutamine derivative to reaction with ammonia (patent document No. 2 and non-patent document No. 3).
Known examples of processes for producing dipeptides which do not comprise D-amino acid as a constituent using an enzyme or a microorganism include a process which comprises subjecting L-amino acid amide and L-amino acid to the action of L-amino acid amidohydrolase (patent document No. 3), a process which comprises subjecting L-amino acid ester and L-amino acid to the action of various microorganisms (patent document No. 4), a process which comprises subjecting L-amino acid ester and L-amino acid to the action of proline iminopeptidase (patent document No. 5), a process which comprises subjecting L-amino acid ester or L-amino acid amide and L-amino acid to the action of an enzyme derived from bacteria belonging to the genus Empedobacter or Sphingobacterium (patent document No. 6), and a process using a protein having the activity to form a dipeptide from one or more kinds of amino acids (patent document No. 7).
Of these processes, the chemical synthesis method tends to involve isomerization of an amino group and production of tripeptides as by-products. For example, according to non-patent document No. 3, crystals of L-alanyl-L-glutamine obtained by repeating recrystallization contained 0.19% D-alanyl-L-glutamine. It is suggested that the enzymatic synthesis method using amino acid ester and amino acid amide as starting materials may possibly form polypeptides consisting of three or more amino acids (patent document No. 6).
There exists, therefore, a demand for dipeptide crystals which do not contain impurities such as a dipeptide comprising D-amino acid as a constituent and a polypeptide consisting of three or more amino acids, and a process for production thereof.
Non-Patent Document No. 1:
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